In a case where electric double-layer capacitors as storage elements are serially connected and then used, conventionally, a life of each storage elements has a large influence on a voltage applied to each storage element. Therefore, when variations in charging voltage of the storage elements occur, a reliability of the life as a whole of the electric storage devices may be degraded, and hence the voltage of each storage element is constantly monitored.
FIG. 9 shows a configuration of conventional electric storage device 20. Electric storage device 20 includes charging/discharging limiting circuit 21. Charging/discharging limiting circuit 21 includes voltage detection unit 22, reference voltage sources 32 and 34, voltage comparison units 36 and 38, charging/discharging limiting switch control unit 40, and charging/discharging limiting switch 42.
Voltage detection unit 22 is provided so as to detect a voltage between plus-side terminal 24P of storage element 24C and minus-side terminal 30M of storage element 30C, the storage elements being connected in a plurality of serial stages. When storage elements 24C, 26C, 28C and 30C are over-charged, the inter-terminal voltages increase themselves, resulting in an increase in voltage of the storage elements as a whole which is generated between plus-side terminal 24P and minus-side terminal 30M. It is to be noted that balance resistances 24R, 26R, 28R and 30R are contacted in parallel with storage elements 24C, 26C, 38C and 30C, respectively.
When these storage elements are over-discharged, the inter-terminal voltages of the storage elements decrease, and the voltage of the storage elements as a whole which is generated between plus-side terminal 24P and minus-side terminal 30M also decreases.
Further, the device is configured such that a lower-limit voltage and an upper-limit voltage, which are allowed during discharging and charging, are set in reference voltage sources 32 and 34 and compared in voltage comparison units 36 and 38 with a detection voltage detected by voltage detection unit 22, and the detection results are separately outputted to charging/discharging limiting switch control unit 40.
Here, the device is configured such that, when the detection voltage detected by voltage detection unit 22 falls below reference voltage source 32, discharging from the storage element is limited through charging/discharging limiting switch 42. When the detection voltage exceeds reference voltage source 34, charging to the storage element is limited through charging/discharging limiting switch 42. It is to be noted that charging is performed by charger 44 and discharging is performed by external load 46.
Moreover, as a prior art document relating to the invention of this application, for example, Unexamined Japanese Patent Publication No. 2000-197277 is known.
However, in a conventional configuration, in a case where an internal resistances (not shown) of storage elements 24C, 26C, 28C and 30C increase, or in a low temperature atmosphere such that an ambient temperature of where the storage elements are placed is not higher than 0° C. assuming that the internal resistance of one storage element among the plurality of storage elements becomes extremely large as compared with the other storage elements, an extra voltage generated by a product of the internal resistance of the storage element and a charging current is applied to the storage element in addition to a voltage obtained by voltage division according to an inverse ratio of the capacity. Therefore, the inter-terminal voltage of the storage elements soon reaches a magnitude of reference voltage source 34, and every time that occurs, the charging current is limited, which causes a problem of taking time for charging.
In order to eliminate such a problem, for example in the low temperature atmosphere not higher than 0° C., it is necessary to take measures such as measuring electric characteristics of the storage elements to perform a selecting operation according to the levels of the characteristics, or increasing the number of serially connected storage elements for securing a resistance voltage. However, it is impossible to say that such measures are the optimum methods since causing a cost increase. Further, the larger the charging current, the more significantly such a tendency appears in the low temperature atmosphere, thereby bringing the electric storage device with a large current capacitance into even more disadvantageous state.